Inherently tacky, elastomeric microspheres have been described for use, for instance, as adhesives in repositionable pressure sensitive adhesive applications. Typically, microsphere adhesives are prepared via suspension polymerization of one or more free-radically polymerizable monomers, usually acrylate or methacrylate monomers, in the presence of surfactants and/or suspension stabilizers. Suspension polymerization is particularly advantageous, since microspheres result directly from the polymerization process as opposed to post-polymerization suspension of an already-formed polymer. Microsphere adhesives described to date comprise homopolymers or copolymers of acrylate or methacrylate monomers, presumably due to the ease with which such monomers can be suspension polymerized in aqueous systems. The preparation of acrylate-based microsphere adhesives is described in, for instance, U.S. Pat. Nos. 3,691,140, 4,166,152, and 5,053,436. Microsphere adhesives prepared from alpha-olefins have not been described.
Non-free radical polymerizations of ethylenically-unsaturated monomers are well known. Typically, these polymerizations use catalysts instead of initiators to effect polymerizations. Examples of such catalyzed polymerizations include Ziegler-Natta (ZN) polymerizations of alpha-olefins, ring-opening metathesis polymerizations (ROMP) of cyclic olefins, group-transfer polymerizations (GTP), and cationic and anionic polymerizations of activated olefins such as styrene or acrylate. More recently, metallocene catalysts have received considerable attention for polymerization of alpha-olefins. ZN and metallocene catalysts for alpha-olefin polymerizations are susceptible to deactivation by adventitious oxygen and water, requiring that such deactivating materials be rigorously excluded from all reagents as well as the reaction vessel.
Recently, the polymerization of ethylene, other olefins, and alkynes using a polymerization catalyst whose cationic portion has the formula EQU LM--R.sup.+
wherein M is a Group VIII metal, L is a ligand or ligands stabilizing the Group VIII metal, and R is H, a hydrocarbyl radical or a substituted hydrocarbyl radical, and a substituted tetraphenylborate anion as the counterion has been described in, e.g. European Patent Application No. 454231. A preferred cationic portion has the formula ##STR1##
wherein L' is a two-electron donor ligand and L" L" are chelating ligands wherein each L" is a neutral two-electron donor ligand, M is nickel or palladium and L' is a two-electron donor ligand or the two L' ligands together are a chelating ligand. Preparation of microspheres and, in particular, microsphere adhesives, was not disclosed.
In disclosures from the same laboratory, Johnson et al., (J. Am. Chem. Soc., 1995, 117, 6414-6415 and supplementary material) describe Pd(II)- and Ni(II)-based catalysts for alpha-olefin polymerizations wherein the catalysts are, for example, cationic metal methyl complexes of the general formula EQU {(ArN.dbd.C(R.sup.1)C(R.sup.1).dbd.NAr)M(CH.sub.3)(OEt.sub.2)}.sup.+ BAr'.sub.4.sup.-
wherein M is Pd or Ni, Ar' is 3,5-C.sub.6 H.sub.3 (CF.sub.3).sub.2, Ar is 2,6-C.sub.6 H.sub.3 (R') where R' is isopropyl or methyl; R.sup.1 is H, methyl, or the two R.sup.1 groups taken together comprise a 1,8-naphthylene-diyl group. The same authors and S. J. McLain et al. reported that the same catalysts copolymerized ethylene and methyl acrylate (see PMSE Abstracts, Vol. 73, p. 458, Fall 1995, Proceedings of the American Chemical Society, Fall 1995, Chicago, Ill.). A full publication describing these findings and a catalyst {(ArN.dbd.C(R.sup.1)C(R.sup.1).dbd.NAr)M(CH.sub.2 CH.sub.2 CH.sub.2 C(OR.sup.2)(C.dbd.O)} .sup.+ BAr'.sub.4.sup.- are reported by Johnson et al. (J. Am. Chem. Soc., 1996, 118, 267-268 and supplementary material), wherein R.sup.2 can be --CH.sub.3, t-butyl, or --CH.sub.2 (CF.sub.2).sub.6 CF.sub.3. Again, preparation of microspheres and, in particular, microsphere adhesives, was not disclosed.